PR18097: Support initializing an _Atomic(T) from an object of C++ class type T
or a class derived from T. We already supported this when initializing
_Atomic(T) from T for most (and maybe all) other reasonable values of T.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214390 91177308-0d34-0410-b5e6-96231b3b80d8